H-bridge with sensing circuit

ABSTRACT

In a cardioverter/defibrillator system, an electrical circuit includes an energy storage device, an output circuit for controlling delivery of pulse therapy from the energy storage device to a patient, and a sensing circuit coupled across the patient to sense the patient&#39;s heart signal. The output circuit may be in the form of an H-bridge switching circuit wherein a pair of switches of the output circuit is simultaneously turned on to discharge residual voltage across the patient that remains after delivery of pulse therapy. Thus, interference with sensing of the patient&#39;s heart signal is avoided.

CROSS-REFERENCE TO CO PENDING AND RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/011,946, filed Nov. 5, 2001 now U.S. Pat. No. 6,865,417, thedisclosure of which is incorporated herein in its entirety.

The present application may find use in systems such as are disclosed inthe U.S. patent application entitled “SUBCUTANEOUS ONLY IMPLANTABLECARDIOVERTER-DEFIBRILLATOR AND OPTIONAL PACER,” having Ser. No.09/663,607, filed Sep. 18, 2000, now U.S. Pat. No. 6,721,597; and U.S.patent application entitled “UNITARY SUBCUTANEOUS ONLY IMPLANTABLECARDIOVERTER-DEFIBRILLATOR AND OPTIONAL PACER,”; having Ser. No.09/663,606, filed Sep. 18, 2000, now U.S. Pat. No. 6,647,292, of whichboth applications are assigned to the assignee of the presentapplication, and the disclosures of both applications are herebyincorporated by reference.

Applications related to the foregoing applications include a U.S.application Ser. No. 09/940,283, filed Aug. 27, 2001 and entitledDUCKBILL-SHAPED IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR CANISTER ANDMETHOD OF USE; U.S. application Ser. No. 09/940,371, filed Aug. 27, 2001and entitled CERAMICS AND OTHER MATERIAL INSULATED SHELL FOR ACTIVE ANDNON-ACTIVE S-ICD CAN; U.S. application Ser. No. 09/940,468, filed Aug.27, 2001 and entitled SUBCUTANEOUS ELECTRODE FOR TRANSTHORACICCONDUCTION WITH IMPROVED INSTALLATION CHARACTERISTICS; U.S. applicationSer. No. 09/940,814, filed Aug. 27, 2001 and entitled SUBCUTANEOUSELECTRODE WITH IMPROVED CONTACT SHAPE FOR TRANSTHORACIC CONDUCTION; U.S.application Ser. No. 09/940,356, filed Aug. 27, 2001 and entitledSUBCUTANEOUS ELECTRODE FOR TRANSTHORACIC CONDUCTION WITH HIGHLYMANEUVERABLE INSERTION TOOL; U.S. application Ser. No. 09/940,340, filedAug. 27, 2001 and entitled SUBCUTANEOUS ELECTRODE FOR TRANSTHORACICCONDUCTION WITH LOW PROFILE INSTALLATION APPENDAGE AND METHOD OF DOINGSAME; U.S. application Ser. No. 09/940,287, filed Aug. 27, 2001 andentitled SUBCUTANEOUS ELECTRODE FOR TRANSTHORACIC CONDUCTION WITHINSERTION TOOL; U.S. application Ser. No. 09/940,377, filed Aug. 27,2001 and entitled METHOD OF INSERTION AND IMPLANTATION OF IMPLANTABLECARDIOVERTER-DEFIBRILLATOR CANISTERS; U.S. application Ser. No.09/940,599, filed Aug. 27, 2001 and entitled CANISTER DESIGNS FORIMPLANTABLE CARDIOVERTER-DEFIBRILLATORS; U.S. application Ser. No.09/940,373, filed Aug. 27, 2001 and entitled RADIAN CURVE SHAPEDIMPLANTABLE CARDIOVERTER-DEFIBRILLATOR CANISTER, now U.S. Pat. No.6,788,974; U.S. application Ser. No. 09/940,273, filed Aug. 27, 2001 andentitled CARDIOVERTER-DEFIBRILLATOR HAVING A FOCUSED SHOCKING AREA ANDORIENTATION THEREOF; U.S. application Ser. No. 09/940,378, filed Aug.27, 2001 and entitled BIPHASIC WAVEFORM ANTI-BRADYCARDIA PACING FOR ASUBCUTANEOUS IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR; and U.S.application Ser. No. 09/940,266, filed Aug. 27, 2001 and entitledBIPHASIC WAVEFORM FOR ANTI-TACHYCARDIA PACING FOR A SUBCUTANEOUSIMPLANTABLE CARDIOVERTER-DEFIBRILLATOR, the disclosures of whichapplications are all hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to defibrillation/cardioversionsystems, and more particularly, to a defibrillation/cardioversion systemhaving an H-bridge with a sensing circuit used in pacing and shockingthe heart.

BACKGROUND OF THE INVENTION

Defibrillation/cardioversion is a technique employed to counterarrhythmic heart conditions including some tachycardias in the atriaand/or ventricles. Typically, electrodes are employed to stimulate theheart with electrical impulses or shocks, of a magnitude substantiallygreater than pulses used in cardiac pacing.

Defibrillation/cardioversion systems include body implantable electrodesthat are connected to a hermetically sealed container housing theelectronics, battery supply and capacitors. The entire system isreferred to as implantable cardioverter/defibrillators (ICDs). Theelectrodes used in ICDs can be in the form of patches applied directlyto epicardial tissue, or, more commonly, are on the distal regions ofsmall cylindrical insulated catheters that typically enter thesubclavian venous system, pass through the superior vena cava and, intoone or more endocardial areas of the heart. Such electrode systems arecalled intravascular or transvenous electrodes. U.S. Pat. Nos.4,603,705; 4,693,253; 4,944,300; and 5,105,810, the disclosures of whichare all incorporated herein by reference, disclose intravascular ortransvenous electrodes, employed either alone, in combination with otherintravascular or transvenous electrodes, or in combination with anepicardial patch or subcutaneous electrodes. Compliant epicardialdefibrillator electrodes are disclosed in U.S. Pat. Nos. 4,567,900 and5,618,287, the disclosures of which are incorporated herein byreference. A sensing epicardial electrode configuration is disclosed inU.S. Pat. No. 5,476,503, the disclosure of which is incorporated hereinby reference.

In addition to epicardial and transvenous electrodes, subcutaneouselectrode systems have also been developed. For example, U.S. Pat. Nos.5,342,407 and 5,603,732, the disclosures of which are incorporatedherein by reference, teach the use of a pulse monitor/generatorsurgically implanted into the abdomen and subcutaneous electrodesimplanted in the thorax. This system is far more complicated to use thancurrent ICD systems using transvenous lead systems together with anactive can electrode and therefore it has no practical use. It has infact never been used because of the surgical difficulty of applying sucha device (3 incisions), the impractical abdominal location of thegenerator and the electrically poor sensing and defibrillation aspectsof such a system.

Recent efforts to improve the efficiency of ICDs have led manufacturersto produce ICDs which are small enough to be implanted in the pectoralregion. In addition, advances in circuit design have enabled the housingof the ICD to form a subcutaneous electrode. Some examples of ICDs inwhich the housing of the ICD serves as an optional additional electrodeare described in U.S. Pat. Nos. 5,133,353; 5,261,400; 5,620,477; and5,658,321, the disclosures of which are incorporated herein byreference.

ICDs are now an established therapy for the management of lifethreatening cardiac rhythm disorders, primarily ventricular fibrillation(V-Fib). ICDs are very effective at treating V-Fib, but are therapiesthat still require significant surgery.

As ICD therapy becomes more prophylactic in nature and used inprogressively less ill individuals, especially children at risk ofcardiac arrest, the requirement of ICD therapy to use intravenouscatheters and transvenous leads is an impediment to very long termmanagement as most individuals will begin to develop complicationsrelated to lead system malfunction sometime in the 5–10 year time frame,often earlier. In addition, chronic transvenous lead systems, theirreimplantation and removals, can damage major cardiovascular venoussystems and the tricuspid valve, as well as result in life threateningperforations of the great vessels and heart. Consequently, use oftransvenous lead systems, despite their many advantages, are not withouttheir chronic patient management limitations in those with lifeexpectancies of >5 years. The problem of lead complications is evengreater in children where body growth can substantially altertransvenous lead function and lead to additional cardiovascular problemsand revisions. Moreover, transvenous ICD systems also increase cost andrequire specialized interventional rooms and equipment as well asspecial skill for insertion. These systems are typically implanted bycardiac electrophysiologists who have had a great deal of extratraining.

In addition to the background related to ICD therapy, the presentinvention requires a brief understanding of a related therapy, theautomatic external defibrillator (AED). AEDs employ the use of cutaneouspatch electrodes, rather than implantable lead systems, to effectdefibrillation under the direction of a bystander user who treats thepatient suffering from V-Fib with a portable device containing thenecessary electronics and power supply that allows defibrillation. AEDscan be nearly as effective as an ICD for defibrillation if applied tothe victim of ventricular fibrillation promptly, i.e., within 2 to 3minutes of the onset of the ventricular fibrillation.

AED therapy has great appeal as a tool for diminishing the risk of deathin public venues such as in air flight. However, an AED must be used byanother individual, not the person suffering from the potential fatalrhythm. It is more of a public health tool than a patient-specific toollike an ICD. Because >75% of cardiac arrests occur in the home, and overhalf occur in the bedroom, patients at risk of cardiac arrest are oftenalone or asleep and can not be helped in time with an AED. Moreover, itssuccess depends to a reasonable degree on an acceptable level of skilland calm by the bystander user.

What is needed therefore, especially for children and for prophylacticlong term use for those at risk of cardiac arrest, is a combination ofthe two forms of therapy which would provide prompt and near-certaindefibrillation, like an ICD, but without the long-term adverse sequelaeof a transvenous lead system while simultaneously using most of thesimpler and lower cost technology of an AED. What is also needed is acardioverter/defibrillator that is of simple design and can becomfortably implanted in a patient for many years.

Typically, ICDs generate an electrical shock by charging a capacitancesystem to a high voltage from a low voltage power source and oscillatorcircuit. Then, the power source is switched out of the circuit and theelectrical charge stored in the capacitance system is discharged throughelectrodes implanted in a patient.

Typical discharge waveforms used with ICDs include monophasic, biphasicor multiphasic waveforms delivered as capacitance discharges. Amonophasic waveform is comprised of a single monotonically decayingelectrical pulse typically truncated before complete discharging of thecapacitance system.

Biphasic waveforms are comprised of a decaying electrical pulse having apair of decaying electrical phases of opposite polarity. To generate abiphasic pulse, an H-bridge switch circuit is used, which is connectedto the implanted electrodes. The H-bridge switches the polarity of thetwo phases. In generating the biphasic pulse, a first phase isdischarged from the capacitance system, similar to a monophasic pulse.When the first pulse is truncated, the H-bridge switch circuitimmediately reverses the discharge polarity of the capacitance system asseen by the electrodes to generate the second phase of the biphasicwaveform being of opposite polarity.

An H-bridge may be used in defibrillators that deliver high voltageelectrical pulses, or shock, and also lower energy pacing pulses to apatient. After a shock or pacing energy is delivered to a patient,normally there is residual voltage on implanted electrodes on thepatient such that the sensing ability of those electrodes is reduced,thus hindering the observation of a heart signal through anelectrocardiogram.

What is needed, therefore, is a defibrillator with an H-bridge switchcircuit such that residual voltage is dissipated from electrodes after ashock or pacing energy is delivered to a patient so that sensingactivity is not affected.

SUMMARY OF THE INVENTION

An electrical circuit for a cardioverter-defibrillation system includesan energy storage device such as a capacitor, an output circuit forcontrolling delivery of defibrillation pulses from the energy storagedevice to a patient, and a sensing circuit coupled across the patient tosense the patient's heart signal. The output circuit may be in the formof an H-bridge switching circuit wherein a pair of switches of theoutput circuit is simultaneously turned on to discharge residual voltageacross the patient that remains after delivery of defibrillation pulses.Thus, interference with sensing of the patient's heart signal isavoided.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference is now made tothe drawings where like numerals represent similar objects throughoutthe figures where:

FIG. 1 is a schematic diagram of a typical ICD circuit including anH-bridge output circuit; and

FIG. 2 is a schematic diagram of an H-bridge with sensing circuitryaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIG. 1, a schematic diagram of a typical ICD circuitincluding an H-bridge output circuit is illustrated. Circuit 10 includesa battery power source 12; a double secondary fly back transformer 15; atransistor switch 14; rectifying diodes 16, 18; high voltage storagecapacitors 20, 22; circuit control 50; an output circuit 30 having fourlegs arranged in the form of an “H” (an “H-bridge 30”), each leg of theH-bridge 30 having switches 32, 34, 36, and 38, respectively; andcardiac electrodes 40, 42.

The H-bridge 30 is connected to cardiac electrodes 40, 42, and is usedto generate a biphasic pulse. The H-bridge 30 switches the polarity ofthe two phases. A first phase is discharged from the high voltagestorage capacitors 20, 22 by activating switches 32 and 38. Then thefirst phase is truncated, and the H-bridge 30 activates switches 36 and34, and reverses the discharge polarity of the high voltage storagecapacitors 20, 22 from the point of view of the cardiac electrodes 40,42, to generate the second phase of the waveform with opposite polarity.

Referring now to FIG. 2, a schematic diagram of an H-bridge with sensingcircuitry according to an embodiment of the present invention isillustrated. An energy storage capacitor 62 is connected to an H-bridge60. A sensing circuit 80 is connected across a patient at nodes 78 and79 of the H-bridge 60.

It should be appreciated that a variety of H-bridge output circuits suchas the one described with respect to FIG. 1 may be used within the scopeof the present invention. Furthermore, it should be noted thatadditional semiconductor switches may be incorporated in each leg of theH-bridge to reduce the voltage that must be switched by each switch.

Although FIG. 2 shows a single energy storage capacitor 62, it iswell-understood in the art that a bank of capacitors may be used, or anyother energy storage device. The energy storage capacitor 62 can becharged to a range of voltage levels, with the selected level dependingon the patient and other parameters. The typical maximum voltagenecessary for ICDs using most biphasic waveforms is approximately 750Volts with an associated maximum energy of approximately 41 Joules. Forsubcutaneous ICDs, the maximum voltages used may be in the range ofabout 50 to about 3150 Volts and are associated with energies of about0.5 to about 350 Joules. The energy storage capacitor 62 may becontrolled to deliver either defibrillation or pacing energy, and couldrange from about 25 to about 200 micro farads for a subcutaneous ICD.

After charging to a desired level, the energy stored in capacitor 62 maybe delivered to the patient in the form of a defibrillation pulse orpacing energy. H-bridge 60 is provided as an output circuit to allow thecontrolled transfer of energy from the energy storage capacitor 62 tothe patient.

Each leg of the H-bridge 60 contains a solid-state switch 64, 66, 68,and 70. Switches 64, 66, 68, and 70 may be silicon controlled rectifiers(SCRs), insulated gate bipolar transistors (IGBTs), or MOSFETs. H-bridge60 further includes electrodes 74 and 76 coupled to a patient.

Switches 64 and 68 are coupled to the positive lead of the energystorage capacitor 62 via bridge line 65. It should be noted that aprotective circuit (not shown) with inductive and resistive propertiesmay be added, for example, at bridge line 65 between the positive leadof the capacitor 62 and the switch 64 to limit current and voltagechanges from the storage capacitor 62 during a defibrillation pulse.Switches 66 and 70 are coupled to the negative lead of the energystorage capacitor 62 via a bridge line 67. The patient is connected tothe left side of the H-bridge by a line 63 and to the right side of theH-bridge by a line 69. Line 63 is connected to electrode 76 and line 69is connected to electrode 74.

By selectively switching on pairs of switches in the H-bridge, abiphasic defibrillation pulse may be applied to the patient. Embodimentsof the present invention may also use monophasic or multiphasicdefibrillation pulses. The switches in the H-bridge are biased with avoltage that allows them to remain turned-on even when conducting lowcurrent.

When the energy storage capacitor 62 is charged to a selected energylevel, the switches 64 and 70 may be turned on to connect the energystorage capacitor 62 with lines 63 and 69 for the application of a firstphase of a defibrillation pulse to the patient. The stored energytravels from the positive terminal of the energy storage capacitor 62 online 65, through switch 64 and line 63, across the patient, and backthrough line 69 and switch 70 to the negative terminal of the capacitor.The first phase of the biphasic pulse is therefore a positive pulse.Before the energy storage capacitor 62 is completely discharged, theswitch 70 is biased off to prepare for the application of the secondphase of the biphasic pulse. Once the switch 70 is biased off, switch 64will also become non-conductive because the voltage falls to zero.

After the end of the first phase of the biphasic defibrillation pulse,switches 68 and 66 are switched on to start the second phase of thebiphasic pulse. Switches 68 and 66 provide a path to apply a negativedefibrillation pulse to the patient. The energy travels from thepositive terminal of the energy storage capacitor 62 on line 65, throughswitch 68 and line 69, across the patient, and back through line 63 andswitch 66 to the negative terminal of the energy storage capacitor. Thepolarity of the second phase of the defibrillation pulse is thereforeopposite in polarity to the first phase of the biphasic pulse. The endof the second phase of the biphasic pulse may be truncated by switchingon switch 64 to provide a shorted path for the remainder of thecapacitor energy through switches 64 and 66. Digital logic (not shown)may be used to control the sequencing of the switches 64, 66, 68, and 70such that the polarity can be inverted so that the first phase isnegative instead of positive. The digital logic generally controls thetiming, the duration of each phase and the inter phase delay.

Sensing circuit 80 is connected to H-bridge 60 across the patient atnodes 78 and 79. Sensing circuit 80 includes a sense amplifier 96 thatsenses differentially and is capacitively coupled across the patient.The sense amplifier 96 has a negative lead connected to node 79 in theH-bridge 60 through a capacitor 82. A resistor 84 is connected tocapacitor 82 between ground and node 81 in a high-pass filter ofapproximately 0.5–20 Hz. Resistor 84 may range in value betweenapproximately 10 KΩ and 500 KΩ. A resistor 92 is connected between node81 and node 103. A capacitor 94 and a resistor 102 are connected inparallel at node 103 as a low pass filter of approximately 30–150 Hz. Itshould be appreciated that there could be multiple low pass filters aswell as multiple high pass filters connected to the negative lead of thesense amplifier 96.

The sense amplifier 96 has a positive lead connected to node 78 via acapacitor 86. A resistor 88 is connected to capacitor 86 between groundand node 87 in a high-pass filter of approximately 0.5–20 Hz. A resistor91 is connected between node 87 and node 99. A capacitor 100 and aresistor 98 are connected in parallel at node 99 as a low pass filter ofapproximately 30–150 Hz. It should be appreciated that there could bemultiple low pass filters as well as multiple high pass filtersconnected to the positive lead of the sense amplifier 96. Furthermore,an embodiment of the sensing circuit may comprise digital logic foroverall control of the sensing circuit.

The sensing circuit 80 allows constant observation of heart signals asan electrocardiogram. When it is time to deliver therapy, a shock orpacing energy is delivered as required. Switches 64, 70, 68, and 66 ofthe H-bridge 60 are sequenced to deliver monophasic, biphasic, ormultiphasic pulses. During shock or even during pacing, as soon as thetherapy pulse is completed, there may be a residual voltage that remainson electrodes 74 and 76 as they are not simply resistors. Capacitancesmay be involved in the patient such that after a pacing pulse ordefibrillation shock there are residual voltages. The residual voltagescould, when present, limit the time that it takes for the differentialsensing amplifier 96 to recover and allow proper continuing observationof the heart signal and determine whether the heart has returned to anormal rhythm or whether there is still an arrhythmia. Thus, theamplifier needs to recover as soon as possible, for example, in muchless than a second, and the voltages have to be within the common modeoperating range of the amplifier as soon as possible.

To improve the post-shock or post-pacing recovery time on theamplifiers, switches 66 and 70 of the H-bridge 60 are turned on at thesame time to discharge any residual voltage across the patient. Byturning on or closing both switches 66 and 70, the voltage across theelectrodes 76 and 74 is effectively shorted out and the residual voltageacross the patient is removed. If there are any capacitances involved inseries or in parallel with the patient, all that energy is dissipated.

After a monophasic, biphasic or multiphasic pacing pulse, or a shock isdelivered, switches 66 and 70 are closed sometime after the end of thepulse, for example, after approximately 50 microseconds to 10milliseconds, for a period of approximately 10 microseconds to up toapproximately a second. This will dissipate the residual voltage acrossthe patient, and improve the recovery time of the sense amplifier.Embodiments of the present invention allow the sensing to be done fromthe H-bridge. To dissipate energy, additional external switches may beused. However, using the switches of the H-bridge itself saves thecomplexity of using external switches.

Numerous characteristics and advantages of the invention covered by thisdocument have been set forth in the foregoing description. It will beunderstood, however, that this disclosure is, in many aspects, onlyillustrative. Changes may be made in details, particularly in matters ofshape, size and arrangement of parts without exceeding the scope of theinvention. The invention's scope is defined in the language in which theappended claims are expressed.

1. An electrical circuit for an implantable cardioverter/defibrillatorcomprising: power storage means connected to a battery; switching meansfor controlling the delivery of defibrillation pulse therapy from thepower storage means to a patient; and sensing means for sensing apatient's heart signal wherein interference with the sensing means isavoided by sequencing the switching means to remove residual voltageacross the patient that results from the defibrillation pulse therapy;wherein the switching means is arranged in an H-bridge output circuitconsisting of first and second high side switches, and first and secondlow side switches, the H-bridge switches configured to produce biphasicdefibrillation pulses, wherein the interference is removed bysimultaneously closing the first and second low side switches.
 2. Theelectrical circuit of claim 1, wherein the power storage means comprisesat least one capacitor.
 3. The electrical circuit of claim 1, whereinthe sensing means further comprises a differential amplifier.
 4. Theelectrical circuit of claim 1, wherein the sensing means furthercomprises at least one low pass filter.
 5. The electrical circuit ofclaim 1, wherein the sensing means further comprises at least one highpass filter.
 6. The electrical circuit of claim 1, wherein the sensingmeans further comprises digital means for controlling the sensing means.7. The electrical circuit of claim 1, wherein the switching meansfurther comprises at least one SCR.
 8. The electrical circuit of claim1, wherein the switching means further comprises at least one IGBT. 9.The electrical circuit of claim 1, wherein the switching means furthercomprises at least one MOSFET.
 10. A method of operating an H-bridgecircuit for delivery of cardiac therapy to a patient from an implantedmedical device, the H-bridge circuit including a first high side switchand a first low side switch defining a first node therebetween forcoupling to a first electrode for therapy delivery, and a second highside switch and a second low side switch defining a second node forcoupling to a second electrode for therapy delivery, the methodcomprising, when the implanted medical device is ready to delivertherapy: closing the first high side and the second low side switches todeliver electrical therapy of a first polarity; and closing the firstand second low side switches to drain any residual voltage from thefirst and second electrodes.
 11. The method of claim 10, furthercomprising, before the step of closing the first and second low sideswitches, closing the second high side and the first low side switchesto deliver therapy of a second polarity.
 12. The method of claim 11,wherein the implanted medical device includes sensing circuitry coupledto the first and second electrodes, wherein the sensing circuitryremains coupled to the first and second electrodes while therapy isbeing delivered.
 13. A method of operating an implantable cardiac rhythmmanagement device, the device including: first and second electrodesconfigured for implantable coupling to a patient; and an H-bridgecircuit including first and second high side switches and first andsecond low side switches, the first high side switch and the first lowside switch defining a first node therebetween for coupling to the firstelectrode, the second high side switch and the second low side switchdefining a second node for coupling to the second electrode, the methodcomprising: closing the first high side switch and the second low sideswitch to deliver an electric pulse of a first polarity to the patient;and closing the first low side switch and the second low side switch toremove residual voltage from the first and second electrodes.
 14. Themethod of claim 13, further comprising, after closing the first highside switch and the second low side switch, but before closing the firstlow side switch and the second low side switch, the steps of: openingthe first high side switch and the second low side switch; and closingthe second high side switch and the first low side switch to deliver anelectric pulse of a second polarity to the patient.
 15. The method ofclaim 14, wherein the device further includes a battery and energystorage circuitry, the method further comprising charging the energystorage circuitry to a predetermined level.
 16. The method of claim 14,wherein the device further comprises sensing circuitry coupled to thefirst and second nodes, wherein the sensing circuitry remains coupled tothe first and second nodes while therapy is being delivered.
 17. Themethod of claim 13, wherein the device further includes a battery andenergy storage circuitry, the method further comprising charging theenergy storage circuitry to a predetermined level.
 18. The method ofclaim 13, wherein the device further comprises sensing circuitry coupledto the first and second nodes, and wherein the sensing circuitry remainscoupled to the first and second nodes while therapy is being delivered.